This paper aims at discussing some aspects of the secrecy characterizing sacred texts by the Ayoreo, a Zamucoan population of the Boreal Chaco in South America, from a linguistic perspective. The magic power exerted by words in Ayoreo traditional culture is the reason why holy texts are kept secret, and this makes it difficult to provide a complete linguistic documentation of Ayoreo ‘hidden’ narratives and ritual formulas. After having outlined etiological myths, the paper will show that language not only can exert a magic power, but that grammar itself, and specifically linguistic gender, has played a role in the development of these sacred narratives. Finally, the paper will compare some elements of Ayoreo culture with lexical data from extinct Old Zamuco, the most conservative language of the family, in order to show that the populations speaking these languages share the same cultural background.